Particle impactor assembly for size selective high volume air sampler

ABSTRACT

Air containing entrained particulate matter is directed through a plurality of parallel, narrow, vertically oriented impactor slots of an inlet element toward an adjacently located, relatively large, dust impaction surface preferably covered with an adhesive material. The air flow turns over the impaction surface, leaving behind the relatively larger particles according to the human thoracic separation system and passes through two elongate exhaust apertures defining the outer bounds of the impaction collection surface to pass through divergent passages which slow down and distribute the air flow, with entrained smaller particles, over a fine filter element that separates the fine particles from the air. The elongate exhaust apertures defining the impaction collection surface are spaced apart by a distance greater than the lengths of elongate impactor slots in the inlet element and are oriented to be normal thereto. By appropriate selection of dimensions and the number of impactor slots air flow through the inlet element is provided a nonuniform velocity distribution with the lower velocities being obtained near the center of the impactor slots, in order to separate out particles larger than a certain predetermined size on the impaction collection surface. The impaction collection surface, even in a moderately sized apparatus, is thus relatively large and permits the prolonged sampling of air for periods extending to four weeks.

The U.S. Government has rights in this invention pursuant to contractNo. DE-AC04-76DP03533 between the U.S. Department of Energy and RockwellInternational Corporation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to surveillance air samplers forsampling particulates in ambient air, e.g., to demonstrate localcompliance with EPA PM-10 regulations by municipalities and industry,and, more particularly, to an improved particle impactor assembly forsize selective fractionating of particulates collected over long periodsof time in high volume air samplers.

2. History of the Prior Art

Surveillance air samplers are widely used to monitor particulate airpollution, in aerosol research studies in general, and for safetymonitoring in the vicinity of industrial plants that process radioactivematerials. Government agencies, especially the United StatesEnvironmental Protection Agency (EPA, hereinafter) specify variousparticulate sampling criteria, generally formulated in terms of theeffective aerodynamic diameter of the collected particulates in air flowpassed through an air sampler. Likewise, the U.S. Department of Energy(DOE, hereinafter) has also enacted guidelines for DOE licensed nuclearfacilities, e.g., 40 CFR 61, part H, due to the highly stringent healthconsiderations for airborne radio nucleides.

In 1984, the EPA developed tentative PM-10 (less than 10 micron particlediameter) criteria for sampling the hazardous fraction of airborne dust,to regulate pollution that comprises particles small enough to bereadily deposited in the human respiratory system. The aforementionedDOE guidelines, however, are also concerned with the recovery ofparticles larger than 10 micron diameters for analysis.

As persons skilled in the art will appreciate, all conventional airsamplers include elements, e.g., screens, to keep out airborne insects,relatively large particles blown about during stormy weather, water inthe form of raindrops or snowflakes, and the like. Basically, suchconventional air samplers convey the air, after such jetsam has beenextracted, generally through a plurality of apertures in an inletelement, to an impactor surface substantially normal to theparticulate-laden air flow or to a cyclone for collection of particlesfor subsequent analysis. The smallest particles are typically capturedon a fine filter surface.

For many analytical purposes, it is highly desirable to have at leastthe fine particulate matter fairly uniformly distributed over the filterelement for X-ray analysis.

It is also known to apply a sticky coating, e.g., a thin layer of asubstance such as petrolatum, commercially available as Vaseline (™), onthe impacted surface to capture the larger particles which otherwise maytend to bounce off. Successive collection stages are often utilized inthe process to fractionate the particles by size.

U.S. Pat. No. 4,461,183, to Wedding, discloses an aerosol sampler inletstructure with performance characteristics that allow collection ofparticles small enough to be inhaled by humans (those having aerodynamicdiameters less than 10 microns) independent of the sampling conditionsof wind speed and direction.

U.S. Pat. No. 4,321,822, to Marple et al, and U.S. Pat. No. 4,211,116,to Pilat et al, are two examples of multistage sampling apparatuses inwhich different stages capture particles in different size ranges. TheMarple et al apparatus includes an element in which apertures aradisposed in spiral configurations, with the impacted surface rotatedadjacent thereto so that the deposit of collected dust is essentiallyuniformly distributed.

U.S. Pat. No. 3,518,815, to McFarland et al, discloses an apparatus inwhich particulate-laden air is provided through a plurality of elongateslots to a rotating impactor surface.

U.S. Pat. No. 4,038,057, to Roth discloses a closed circuit air samplerin which air is conveyed through pluralities of radially disposedapertures to an impactor surface enclosed in a frangible container, sothat removal of the deposited dust requires destruction of the otherstructure to prevent reuse and possible contamination, deliberate oraccidental, in successive uses of the device.

U.S. Pat. No. 3,681,973, to Ludwig, discloses a sampler structuremounted for rotation in a plane perpendicular to the wind direction andhaving an entrance slot communicating with a hollow aerodynamicallyshaped chamber lined at its internal surface with a collection paper forcollecting the particles along the length thereof, the impingementpoints of differently sized particles being determined by thetrajectories that the particles follow within the chamber, depending ona combination of wind speed, particle diameter, and centrifugal forcerelated to the speed with which the sampler slot is rotated.

Although known air samplers typified by the ones discussed above arecapable of separating particulates in the below 10 micron range fromthose that are larger, they are expensive, complicated to operate, poseproblems in handling of the separate collection elements, and do notallow long term operation, e.g., for periods extending for up to fourweeks at a time.

There is, therefore, a long felt and serious need for an improvedsurveillance air sampler for sampling ambient air for long periods oftime, which is relatively inexpensive and easy to operate, which isreadily adaptable to separate particles having aerodynamic diameterslarger than 10 microns from those that are smaller, and which allows foreasy recovery of both fractions.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide an improvedparticle impactor assembly, suitable for use with conventional airsampling apparatuses, for size selective collection of particulatematter entrained in the sampled air.

It is another object of this invention to provide an improved particleimpactor assembly, suitable for use with conventional air samplerapparatuses, that will permit air sampling over a number of weeks.

It is a further object of this invention to provide an improved particleimpactor assembly that is readily adapted to separate out particles ofpredetermined size from those that are smaller, during long durationsampling of particulate laden air.

It is an even further object of this invention to provide an improvedparticle impactor assembly suitable for use with a size selective, highvolume, long duration air sampling operation that allows effectivecollection and recovery of particles up to 70 microns aerodynamicdiameter.

It is also an object of this invention to provide a method forfractionating by size the collection of airborne particulate matter fromair sampled over long periods of time by controlling the velocitydistribution of the sampled air directed at a dust collecting impactorsurface.

These and other related objects of this invention are achieved, in apreferred embodiment, by directing the air flow in an air samplerapparatus through a particle impactor assembly having an inlet elementwith a flat horizontal upper portion with a plurality of similarparallel slot-like inlet apertures, referred to hereinafter as impactorslots, each of which has a length V and width U. The inlet element alsohas a downwardly depending rim of height H. A flat impactor plate issealingly affixable to the inlet element of this rim, and has in it apair of similar parallel slot-like exhaust apertures having a width S,oriented normal to the impactor slots thereabove. At the upper surfaceof the impactor plate the exhaust apertures each have a length thatextends past the last adjacent inlet slot by a distance of at least 2U.The rim portions of the inlet element that parallel the ends of theimpactor slots are at a distance R therefrom, and at the upper surfaceof the impactor plate the exhaust apertures are separated by a distanceof at least (V+2U). This geometry causes collection on the impactorplate of particles larger than 5 um with increasing efficiency, e.g. 50%efficiency at 10 um, the smaller particles tending to flow with the airpassing through the exhaust apertures for separate filtration. In oneaspect of this preferred embodiment, the exhaust apertures havediverging sides that slow down and distribute the flow to allow evenlydistributed collection of the finer particles on a fine filter disposedtherebelow.

Still other objects and advantages of the present invention will becomereadily apparent to those skilled in this art from the followingdetailed description, wherein only the preferred embodiment of theinvention is shown and described, simply by way of illustration of thebest mode contemplated of carrying out the invention. As will berealized, the invention is capable of other and different embodiments,and its several details are capable of modification in various obviousrespects, all without departing from the invention as disclosed herein.Accordingly, the drawings and description of the invention are to beregarded as illustrative in nature, and not as restrictive.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the inlet element located above theimpactor plate in the apparatus of the present invention. An x-y-zCartesian reference system is provided for the convenience of subsequentreference.

FIG. 2 is a vertical cross sectional view of the inlet element atsection A--A, i.e., in the x-z plane.

FIG. 3 is a vertical cross sectional view of the impactor plate atsection A--A, i.e., in the x-z plane.

FIG. 4 is a vertical cross sectional view of the inlet element inposition above the impactor plate at section B--B, i.e., in the y-zplane.

FIG. 5 is a vertical cross sectional view of portions of an inletelement and an impactor plate, in normally operative juxtaposition, astaught in the prior art, in the y-z plane.

FIG. 6 is a vertical cross sectional view, in the x-z plane, of theparticle collecting elements according to a preferred embodiment of thisinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is intended to provide improved results in theoperation of known air sampling apparatuses over prolonged periods, and,especially, for the separation thereby of airborne particles into twosize fractions according to the human thoracic particle size separationat a nominal 10 micron aerodynamic diameter. This cut is different thanthe much sharper cut produced by a conventional impactor set at a 10micron cut-point. It is to be understood that the normal practice ofensuring the elimination of flying insects, uninteresting and undesiredjetsam, and water is obtained by conventional means. The particle-ladenair that passes through the combination of the preferred embodiment ofthis invention likely will be flowing at a rate within a predeterminedrange, e.g., approximately 40 cfm or an impactor slot velocity ofapproximately 13 ft/s (4 m/s); over prolonged periods.

Referring now to FIG. 1, it should be noted that a reference Cartesiancoordinate system is indicated by the triad x-y-z aligned with theprincipal sides of the generally rectangularly shaped elementsillustrated therein.

The assembly 10 of FIG. 1 illustrates, in perspective view, an inletelement 12 placed on the upper surface of impactor plate 14. Purely forreference purposes, the longer sides of inlet element 12 and impactorplate 14 are aligned with the y axis and their respective widths arealigned with the x axis. Inlet element 12 is provided with a pluralityof elongate impactor slots 16 through which air passes in a downward,i.e., z axis, direction. Inlet element 12 is also provided with twocylindrical apertures 18 through which are passed threaded bolts 20 usedwith washers and nuts (not shown) to affix together inlet element 12 andimpactor plate 14 at a common contact plane. Thus, any air flowingdownward through impactor slots 16 must pass through the two exhaustapertures (to be described hereinafter) provided in the impactor platetherebelow.

As illustrated in FIG. 2, inlet element 12 has a generally planar uppersurface provided with similar elongate impactor slots 16 which areoriented to be parallel to each other. These are conveniently formed bya milling operation that causes their ends to be rounded cylindrically.The exact shape of the ends of impactor slot 16 is not critical tosuccessful operation of the present invention. Impactor slots 16 in thepreferred embodiment all have the same width U and same length V and areoriented to be in the x-z plane. Depending downwardly from the planartop portion of inlet element 12 by a distance H is a rim 17 around theperiphery of inlet element 12. The inner vertical wall surfaces of rim17 in the x-z plane are preferably parallel to each other and areseparated by a distance R with respect to both ends of impactor slots16. Thus, the separation between the inner vertical surfaces of rims 17in the x direction is (V+2R). It is believed that for collection ofparticles in the range 10-70 microns the distance R should be not lessthan the width U of impactor slots 16. The height H of the rim 17 withrespect to the lower surface of the upper planar portion of inletelement 12 is likewise a design parameter, just like the dimensions U, Vand R, and all of these dimensions may be altered to suit the preciseneeds to be met by the overall air sampling system.

As best seen in FIG. 3, impactor plate 14 is a generally planar elementat least as large in size as inlet element 12 at their common contactingsurface and is provided with two elongate slot-like exhaust apertures 22oriented to be in the y direction with respect to the referencecoordinate system. In other words, exhaust apertures 22 are normal toimpactor slots 16. Impactor plate 14 is provided with cylindricalapertures 24 through which are passed bolts 20. Each exhaust aperture 22at the upper surface (at which contact is made with the inlet element12) has a width S and the adjacent edges of parallel exhaust apertures22 are separated by a distance W, which equals (V+2U).

Each of the exhaust apertures 22 has a length that preferably extendspast the last adjacent impactor slot 16 by a distance at least twice theimpactor slot width U, i.e. 2U, and is preferably formed to have slopingsides so that the exhaust aperture offers a diverging air passage in thedownward or streamwise direction. Since the air flow through the exhaustapertures 22 is in the subsonic region under all operating conditions,such a divergence of the air flow passage through exhaust apertures 22will cause the air flow to diverge laterally and simultaneously slowdown. The consequences of this are discussed more fully hereinafter.Each of the sloping sides of exhaust apertures 22 preferably forms anangle α with the local vertical as shown in FIG. 3.

The heads of bolts 20 are located in recesses provided therefor inimpactor plate 14. As shown in FIG. 4, planar contact between the uppersurface of impactor plate 14 and rim 17 of inlet element 12 defines acollection surface 28 which in the y direction has the dimension Z.Although not clearly visible in FIG. 4, both of the exhaust apertures 22are positioned below and symmetrically parallel to the plane passingthrough the axes of bolts 20 as well as the midpoints of impactor slots16 thereabove.

FIG. 5 is an illustrative depiction of a known inlet/impactor plateassembly, in vertical cross section normal to the impactor slots 36 ofan inlet element 32 positioned parallel to and separated from animpactor plate element 34 having a plurality of elongate surfaces 38that are impacted by particles and where some of the particles arecollected.

The principal components that work together and provide the improvementaccording to a preferred embodiment of this invention are best seen inFIG. 6. For convenience, impactor plate 14 and inlet element 12 areindicated as having the same outside dimensions of length and width inthis figure. Persons skilled in the art will naturally be able tocontemplate, design and utilize elements as taught in this disclosureregardless of the important dimensions of the contacting elements.Starting from the top, inlet element 12 contacts the top surface ofimpactor plate 14 which, in turn, contacts the top surface of aperipheral spacer element 52. Directly beneath spacer element 52 and incontact therewith is a thin, usually rather fragile, typicallyfiberglass paper-like filter element 48 resting on and supported by afine wire screen 50 that is conveniently made of stainless steel. Theouter edges of filter element 48 and screen 50 are sandwiched betweenthe lower surface of spacer element 52 and the upper periphery of atapered guide element 54 that has two long sides having taperedpositions 56 and vertically oriented portions 58 to guide the air flowafter the dust particles have been extracted therefrom by impactioncollection of larger (typically more than 5 micron) particles onimpactor plate 14 and the finer particles on filter element 48.

The essential geometry having been explained, it now becomes possible toexplain the reasons for and the benefits obtained by the particularconfigurations selected for the various elements of the presentinvention. For this purpose, it is helpful to compare the known geometryillustrated in FIG. 5 and the improved geometry according to a preferredembodiment of the present invention as illustrated in FIG. 6. Note thatthe air flow downward through impactor slots 36 of the prior art (FIG.5) has an essentially uniform velocity distribution along the length ofeach elongate impactor slot 36. This is indicated by downwardly directedequally sized arrows collectively identified by the numeral 44 in FIG.5. The dust collecting areas 38 of impactor plate 34 of the prior artare disposed to be directly beneath corresponding impactor slots 36 andare relatively narrow, the distance between successive openings, i.e.exhaust apertures 35, on impactor plate 34 being characterized by theletter M. This is the same spacing as between adjacent impactor slots36.

As a direct consequence of this selection of relative dimensions, therelatively uniform velocity distribution 44 of the particulate laden airflow through each impactor slot 36 corresponds to dust collection on oneof the surface areas 38 having a width M directly therebelow. Relativelylarge and heavy dust particles 40 hit surface 38 and, in most instances,are collected thereon. The flow of air, however, has to divert to eachside of each surface 38 to pass through the exhaust apertures 35 ofimpactor plate 34. This diversion of the air flow direction causes acentrifugal force further tending to throw the larger particles 40toward impactor collection surface 38, but the samller dust particles 42are light enough to follow the streamlines of the air flow andsubstantially pass between adjacent collection surface strips 38, asindicated by arrows pointed downward in FIG. 5. Experience has shownthat the provision of a relatively uniform velocity distribution 44through impactor slots 36, causes a sharp cut-off for the collection ofparticles larger than 10 microns, for example, instead of the moregradual rejection of particles near 10 microns, so that the desiredfractionating effect of separating out particles larger than those thatare likely to enter the human respiratory system is not adequatelyrealized. The relatively narrow collection surfaces 38 can causeinsufficient capture of larger particles, because proper, routineadhesive application to hold the large particles is difficult. Alsoparticle recovery is impeded by the discontinous surfaces 38.Additionally, the narrow collection surface has a limited dust holdingcapacity of days versus a month.

By contrast to the prior art as illustrated in FIG. 5 and explained inthe immediately preceding paragraph, as best understood with referenceto FIG. 6, the preferred embodiment of the present invention causes airflow 46 through each typical impactor slot 16 to head toward acollection surface 28 that is wider than the length of impactor slots16, and this results in a slowing down of the air flow near the centerof each impactor slot 16. This is illustrated in FIG. 6 by arrows ofunequal length, symmetrically disposed about the center of a typicalimpactor slot 16 and collectively identified by the numeral 46. Aspersons skilled in the art will immediately appreciate, the consequenceof a downward flow of air through a typical impactor slot 16 toward aclosely adjacent surface such as impact collection surface 28 is tocause an increase in flow resistance toward the center of the impactorslot between the lower surface of inlet element 12 and impactor surface28. As a consequence, only relatively large particles 40 impact thecentral area of collection surface 28 and are collected thereon whilesome of the smaller particles are collected on the surface of 14 nearthe edges of exhaust aperatures 22 and away from the central area of 14.To each side of collection surface 28 the diverted air flow (indicatedby curved arrows) entrains the smaller particles and carries themthrough exhaust apertures 22. As with the prior art, as the air flow isturned to pass the ends of impactor slots 16 and incidental centrifugalacceleration is imposed on the larger particles, tending to throw themtoward surface 28 of impactor plate 14 while the lighter particles 42continue entrained in the air flow and tend to follow the streamlinesthrough exhaust apertures 22.

The air flow through exhaust apertures 22 is a divergent one asindicated by curved arrows in FIG. 6. This flow through exhaustapertures 22 slows down and distributes itself across the region beneathimpactor plate 14 and the upper surface of filter element 48. As adirect result of this slowing down and redistributing of the air flowafter passage through the divergent exhaust apertures 22, the flowthrough filter element 48 is relatively slow and results in a generallyeven distribution of the finer particles 42 on filter element 48. Theair flow 60 having passed filter element 48, and through the openings instainless steel screen 50, passes downward between inwardly taperingwalls 56 and downward walls 58 for rejection through the ambientatmosphere or as desired.

To summarize the distinctions and the advantages obtained by thegeometry as taught herein over the closest prior art geometry forcomparable elements, it must be noted that by providing the inlet airflow through a plurality of relatively narrow elongate impactor slots 16to a relatively large collection surface 28 of impactor plate 14 anonuniform velocity distribution in the streamwise direction is obtainedwhich tends to deposit the larger particles with varying efficiencyalong the length of impactor slots 16 on impactor surface 28, and thefurther provision of divergent elongate exhaust apertures 22 in impactorplate 14 slows down and distributes the air flow with entrained smallparticles 42 over a filter element 48 supported on a wire mesh screen 50of conventional construction. Because the impactor collection surface 28has a lateral dimension W that exceeds length V of impactor slots 16, byjudicious selection of height H of the undersurface of inlet element 12above impactor surface 28 it becomes possible to obtain the optimumnonuniform velocity distribution 46 for a selected operational flow ratethrough the sampler, with the intended advantage of effective separatecollection on surface 28 of the larger particles 40, e.g., particleslarger than 10 microns according to the human thoracic collectionsystem, from the incoming air flow. The deliberate divergence of the airflow with smaller entrained particles 42 through elongate exhaustapertures 22 thereafter effects a uniform distribution of the finerparticles 42 over filter element 48.

Naturally, as persons skilled in the art of fluid dynamics and particlemechanics will immediately appreciate, the desired air flow willdetermine various dimensions discussed herein, the number of impactorslots 16, the type and rating for filter element 48, and the like. Alsoimportant in this respect are other parameters such as ambienttemperature and relative humidity which are bound to be different,depending on the location and weather where the air sampler is used. Inbrief, because numerous such operational parameters have to beconsidered by a designer and a user for a particular sampling operation,it may become highly desirable to vary certain geometric parameters ofthe combination taught herein.

Although not discussed in detail and not illustrated in FIG. 6 for thesake of simplicity, it will be appreciated by persons skilled in the artthat the different components may be held together by clamping, bolting,or other conventional means and that such means may be selected to havethe flexibility to accommodate changes in the geometric dimensions ofthe various elements that coact together. Thus, for a given inletelement 12, in order to accommodate to lower air flows, it should berelatively simple for a person skilled in the art to block off one ormore impactor slots 16, or to replace a given inlet element 12 by adifferent inlet element 12 having a different dimension H, or to replacea given impactor plate 14 by a different impactor plate 14 havingexhaust apertures 22 of a different width S, a collection surface 28 ofa different length Z, or differently diverging exhaust apertures 22having different values of the angle α or to replace a given spacer 52by another spacer of a different thickness or a different width, or thelike. In other words, depending on the particular exigencies of a givenapplication, a user can adapt the disclosed combination with greatfreedom and a manufacturer of such elements may find it worthwile andprofitable to provide a range of components that are all capable ofcoacting together in different combinations within the scope of theinvention disclosed herein.

It is believed that a nonuniform velocity distribution through theimpactor slots 16, which can be altered by selection of different widthsU and different lengths V for these apertures, by itself provides aready control over the size distribution of particles that are collectedon impactor collection surface 28 therebelow. This controlled nonuniformvelocity distribution, deliberately obtained by disposing impactor slots16 orthogonal to more widely spaced apart elongate exhaust apertures 22in the impactor plate, results in a unique, nonobvious and highlyversatile collection mechanism for separating out particulate matterhaving aerodynamic diameters greater than, for example, 10 micronsaccording to the thoracic system.

As will be appreciated by persons skilled in the art, the relativelylarge size of the impactor collection surface 28 permits collection overa prolonged period which experimental evidence indicates may be as longas four weeks. As is well known in the art, the collection surface 28may be coated with an appropriate adhesive, e.g., petrolatum orVaseline™, that may subsequently be rinsed off with a solvent such aschlorothene™, for recovery of the collected dust for further analysisand assay thereof. Because the collection surface 28 according to thepresent invention is relatively large, the very act of coating it withan adhesive is made much easier and more convenient than is the casewith the narrow surfaces as taught in the prior art (best illustrated inFIG. 5).

Because the important components of the present combination are easilyseparable, contact between adjacent parts being effected at smoothplanar surfaces only, it is relatively easy even for unskilled personnelusing simple tools to extract impactor plate 14 and filter element 48from the disclosed assembly for replacement thereof for a furthersampling activity.

The method for using the present invention is relatively straightforwardand simple. A clean impactor plate 14 is coated with a suitable adhesiveover its principal large particle collecting surface 28 and is bolted toa suitable inlet element 12. The two of them in combination are thenplaced over a fresh unused filter element 48 with a spacer 52therebetween, and the assembly is bolted, clamped or otherwise completedfor use. The combination according to FIG. 6 is then utilized with aconventional air sampling system in which particulate laden air isdirected through impactor slots 16 as described hereinabove. Theavoidance of cyclone separators, electric motors to turn collectionsurfaces, more complex assemblies, frangible outside containers, andother such disadvantages of the prior art make the present inventionrelatively simple and easy to use. After a suitable period of operation,the collected dust may be removed on the impactor plate 14 (largerparticles) and filter element 48 (smaller particles). The variousdisassembled elements may be readily cleaned with a solvent and reusedrepeatedly.

It is anticipated that persons skilled in the art, armed with theknowledge provided by this disclosure, will contemplate a variety ofmodifications in the structure and uses of this invention. All suchmodifications and variations are expressly contemplated as beingencompassed within the claims appended below.

What is claimed is:
 1. A particle impactor assembly, for use in a highvolume air sampler apparatus for collecting and separating by size,entrained particles from sampled air, comprising:a. an inlet elementformed to have a horizontal and substantially flat upper portion with aplurality of similar elongate parallel through impactor slots, eachhaving a length V and a width U, said upper portion having:(1) adownwardly depending peripheral rim of height H, having inner verticalsurfaces, and (2) a distance R between each impactor slot end and thenearest rim portion normal to said impactor slot end, such that theseparation between the inner vertical surfaces of said rim portionextending parallel to said length V equals at least (V+2R); b. animpactor plate having an upper surface sealingly affixable to alowermost part of said rim of said inlet element, the impactor platebeing formed to have two similar elongate parallel through exhaustapertures symmetrically oriented normal to the impactor slotsthereabove, said exhaust apertures being separated by a distance of atleast (V+2U) and each said exhaust aperture at the upper surface havinga width S and a length extending past the corresponding last adjacentimpactor slot by a distance of at least 2U, for impaction thereon of atleast the larger particles entrained in the air flow, whereby thevelocity distribution in the air flow through said impactor slots isnon-uniform and has low velocities near a central portion of saidimpactor slots for promoting the impaction of larger particles only andthe collection of both larger and smaller particles on said impactorsurface below said impactor slots, where air flows through saidapertures; and c. means for affixing said inlet element to said impactorplate.
 2. A particle impactor assembly according to claim 1,wherein:said exhaust apertures diverge through the thickness of saidimpactor plate.
 3. A particle impactor assembly according to claim 2,wherein:said diverging exhaust apertures have lengthwise inclinedsurfaces disposed at a predetermined angle α to a local normal to saidimpactor plate upper surface.
 4. A particle impactor assembly accordingto claim 3, wherein:

    α=45°.


5. A particle impactor assembly according to claim 1, wherein:saidimpactor plate upper surface, at least between said exhaust apertures,is coated with an adhesive material to facilitate collection thereby ofsaid particles impacting thereon.
 6. A particle impactor assemblyaccording to claim 5, wherein:said adhesive is separable from saidcollected particles by a solvent.
 7. A particle impactor assemblyaccording to claim 6, wherein:said dissolvable adhesive comprisespetrolatum.
 8. A particle impactor assembly according to claim 6,wherein:said dissolvable adhesive comprises a heavy oil.
 9. A particleimpactor assembly, according to claim 1, wherein:said distance R is atleast equal to said width U of said inlet slots.
 10. A particle impactorassembly according to claim 1, further comprising:a peripheral spacerelement sealingly affixable to a lower surface of said impactor plate.11. A particle impactor assembly according to claim 10, furthercomprising:filter means, sealingly affixed to a lower surface of saidspacer element for disposition substantially parallel to said lowersurface of said impactor plate, for filtering fine particulatesentrained in air flowing through said exhaust apertures.
 12. A particleimpactor assembly according to claim 11, wherein:said filter meanscomprises a fine filter element selected to filter out particles withina predetermined size range.
 13. A particle impactor assembly accordingto claim 12, wherein:said filter means further comprises a filterelement support means for supporting said fine filter element thereon,said support means being foraminous to enable ready flow of filtered airflow therethrough.
 14. A particle impactor assembly according to claim4, wherein:said impactor plate upper surface, at least between saidexhaust apertures, is coated with an adhesive material to facilitatecollection thereby of said particles impacting thereon.
 15. A particleimpactor assembly according to claim 14, wherein:said adhesive isseparable from said collected particles by a solvent.
 16. A particleimpactor assembly according to claim 15, further comprising:a peripheralspacer element sealingly affixable to a lower surface of said impactorplate; and filter means, sealingly affixed to a lower surface of saidspacer element for disposition substantially parallel to said lowersurface of said impactor plate, for filtering fine particulatesentrained in air flowing through said exhaust apertures.
 17. A methodfor sampling particulate matter entrained in a directed air flow, suchthat particles larger than a predetermined characteristic size arecollected separately from smaller particles according to the thoraciccut, comprising the steps of:a. directing the particulate-laden air flowdownwardly through a plurality of similar elongate, parallel impactorslots of a first length V and a first width U; b. diverting saidparticulate-laden air flow coming through said impactor slots by animpactor surface disposed a predetermined distance therebelow, the airflow being then directed through two elongate, parallel exhaustapertures in said impactor plate, the exhaust apertures being separatedby a distance of at least (V+2U) and being oriented normal to theimpactor slots whereby the velocity distribution in the air flow throughsaid impactor slots is non-uniform and has low velocities near thecenters thereof for promoting only the impaction and collection of thelarger of said particles on a central area of said impactor surface; andc. filtering the air directed through said exhaust apertures by passagethereof through a filter to collect the finer of said particlesseparately thereby.
 18. A method according to claim 17, furtherincluding the step of:diverging the air flow through said exhaustapertures to slow down the same and to uniformly distribute thecollection of said filtered fine particles.
 19. A method according toclaim 18, further including the step of:coating said impactor surfacewith an adhesive material.
 20. A. method according to claim 17, furtherincluding the steps of:periodically removing collected particles fromsaid impactor surface and from said filter.